1. Field of the Invention
This invention relates to resins having enhanced adhesion to metal. In particular, this invention relates to curable episulfide and episulfide/epoxy resins having enhanced adhesion to metals such as copper, gold, and the like. These resins find particular utility in the formation of circuit board laminates, as protective coatings and adhesives.
2. Description of the Related Art
Epoxy resins are widely used for protective coating, electrical, and adhesive applications, due to their high chemical and corrosion resistance, good mechanical and electrical properties, and strong adhesion to many different substrates, especially metals.
The electronics industry depends upon fiberglass reinforced epoxy resin circuit boards. These circuit boards comprise a fiberglass reinforced epoxy resin dielectric layer adhered to an electrically conductive layer, typically copper. Technological advances have resulted in increasing demands on circuit boards. Properties such as dimensional stability, residual stresses and coefficient of thermal expansion (CTE) are particularly important. Additionally, the fiberglass reinforced epoxy resin dielectric layer must remain adhered to the electrically conductive layer, as separation of the dielectric layer from the electrically conductive layer results in board failure.
Because metals are subject to corrosion problems, many protective treatments have been developed, such as metal plating, chromate, corrosion inhibitors, and coupling agents. Some of these treatments also promote adhesion of another substance to the metal. Protective treatments are usually applied before the metal is used. In general, these corrosion-resistance and/or adhesion-promoting treatments are relatively expensive, complicated and in some instances, harmful to the environment. Epoxy resins are an attractive alternative to the previously mentioned treatments and are widely used for protective coatings to provide corrosion protection to metals.
Typical epoxy resins show excellent initial adhesion to several common metals but adhesion after water exposure is poor. In circuit boards this loss of adhesion results in board failure. In protective coatings the loss of adhesion means the loss of protection. Additionally, one of the most widely used epoxy/curative systems, epoxy/DICY, has a relatively long pot life and prepreg storage life but an undesirably high cure temperature and long cure time. As a result, an accelerator is typically added to reduce the cure temperature and time but the accelerator concurrently reduces the pot life and prepreg storage life. As a result the prepreg must be stored below room temperature and in low humidity conditions. Special storage conditions results in increased manufacturing costs.
Accordingly, there remains a need in the art for resin systems with more durable metal adhesion, long pot life and long prepreg storage life.
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a metal-polymer composite comprising a metal and substrate layer disposed on the metal, wherein the substrate layer is formed from a curable thermosetting resin system comprising a curing agent and about 0.1 to 100 weight percent (wt %), preferably about 0.2 to about 80 wt %, more preferably about 0.4 to about 60 wt %, and most preferably about 2 to about 40 wt % of an episulfide, and further wherein the resin system is cured in the presence of the metal. In another embodiment, the thermosetting system comprises about 0.1 to about 50 wt %, preferably about 1 to about 40 wt % and more preferably about 2 to about 30 wt % of at least one episulfide resin, at least one epoxy resin reactive therewith, and a curing agent, wherein the resin system is cured in the presence of a metal, such as a layer of copper or gold. A preferred epoxy resin is the diglycidyl ether of bisphenol A, which is the condensation product of bisphenol A and epichlorohydrin (hereinafter abbreviated xe2x80x9cDGEBAxe2x80x9d). A preferred episulfide resin is the episulfide analog of the preferred epoxy resin.
The two aforementioned thermosetting resin systems find particular utility as protective coatings and adhesives for metals, as well as cured compositions, preferably laminated to metals, to form, for example circuit boards.
The above discussed and other features and advantages will be appreciated and understood by those skilled in the art from the following detailed description and drawings.